It does sound as though the soil has settled. You can build it up by spreading topsoil over your lawn. It might take the entire summer, depending upon how high you need to raise it. Here's how: spread slightly less than one inch of topsoil over the grass. The grass will grow through the topsoil. When it has and you've mowed it a couple of time, spread another three quarters of an inch of topsoil over the lawn. Wait for it to grow up through the topsoil. Repeat as necessary to build up the soil.

An alternative is to rent a sod cutter and remove your existing lawn in long strips - like sod. Roll or lay them in a shady spot and keep them moist. Spread the required amount of topsoil over the bare earth to raise the lawn area, then re-lay your strips of lawn. This method will be faster than the first method but either should result in a lawn that's higher than it is now. Best wishes with your project.